HELP

I am a level 18 embermage and throughout the entire game so far I have had a lot of trouble keeping up with the area levels. Everytime I go to a new area it is always a higher level than I am and it gets really frustrating. I kill all the enemies I can on the previous area but I am still too low of a level. So now everytime I progress into a new area I can never use any of the loot there. Also I am on casual difficulty. Please help!!